# Lewis number in docs?

 Register Blogs Members List Search Today's Posts Mark Forums Read

 January 25, 2008, 13:09 Lewis number in docs? #1 Patti Guest   Posts: n/a I'm looking for references in the Phoenics documentation about the molecular diffusivity models. Specifically, I want to know if the "unity Lewis number" approximation is used, and, if so, how to set the Lewis number to less than unity for very light species. Can anyone please point me in the right direction? Thank You Very Much, Patti

 January 29, 2008, 17:10 Re: Lewis number in docs? #2 Joe Guest   Posts: n/a From my understanding of the documentation, I think unity Lewis number is used. The setting you're looking for is PRNDTL(phi) and PRT(phi). In the documentation they are stated as the ratio of kinematic viscosity divided by diffusion coefficient, in the case of species. So that would be the Schmidt number, Sc = Le*Pr. PRNDTL(phi) defaults to 1.0 for most variables. I hope that helps

 February 7, 2008, 21:23 Re: Lewis number in docs? #3 Patti Guest   Posts: n/a Thank you very much, Joe. That is not good to find out but good to know. The Lewis number for H2 should be about 0.3 and that for H ~0.1. I hope I can figure out how to set these. I've been digging through the encyclopedia, but so far no joy for the Schmidt or Lewis numbers. Patti

 February 8, 2008, 11:45 Re: Lewis number in docs? #4 Joe Guest   Posts: n/a Look for PRNDTL in the Phoenics encyclopedia, or PRT. In the Phoenics instruction language, assigning Prandtl or Schmidt numbers is done with the command PRNDTL(phi)=X. When phi is TEM1 or H1, you're setting a Prandtl number, when the phi is a mass fraction like H2, you're setting the Schmidt number. If you are simulating combustion of H2, hydrogen, with constant properties, in Phoenics you might set PRNDTL(H1 or TEM1) = 0.7; this is the Prandtl number. Then you would give the Phoenics command PRNDTL(H2)= 0.3*0.7, setting the Schmidt number for H2 to Le * Pr.

 February 9, 2008, 16:38 Re: Lewis number in docs? #5 Patti Guest   Posts: n/a Thank you so much, Joe! I might be able to try that. Normally I set PRNDTL(TEM1)=-GRND9 so CHEMKIN to calculate accurate thermal conductivities of my gas mixture. To try your suggestion I'd need to retrieve the value it calculated (say, VAL) then set PRNDTL(H)=0.1*VAL. (Perhaps CHAM can comment here on how to do this in version 3.4?) The CHEMKIN subsystem docs show that I can calculate species diffusivities by PRNDTL(H)=-GRND9, but if I can explicitly set individual species diffusivities, how is it that the Unity Lewis Number assumption is active? So far all I really know is that if I set PRNDTL(H)=-GRND9 my results are VERY different from not setting it, seem physically unreasonable (compared to experiment), and I'm hoping for an explanation from CHAM. THANKS!! Patti

 February 11, 2008, 16:08 Re: Lewis number in docs? #6 Joe Guest   Posts: n/a If you're using GRND9 for density, ENUL, PRNDTL(TEM1), and PRNDTL(species), then you would be using Chemkin's transport libraries to calculate properties, so the case wouldn't be using a unity Lewis number assumption at that point. It would (essentially) have an individual Lewis number for each specie in each cell, based on the tranport properties Chemkin calculates. Possibly you could get Phoenics to store the local Lewis number for you using In-form. Definitely using some Fortran programming in Ground would do it. That might let you figure out why you're getting unreasonable results.

 May 6, 2008, 15:35 Re: Lewis number in docs? #7 PattiMichelle Sheaffer Guest   Posts: n/a Thanks for your help! I was able to get everything operating correctly (version 3.4) except for the Soret effect. The Enhanced Diffusion model seems to work OK. Now I notice in the Phoenics2006 code that the Enhanced Diffusion model was broken (i.e., in version 3.4), but apparently fixed in Ph2006. So I'm in the process of trying to get my models ported to 2006. The downside is that the Soret effect seems to be still borken. THANKS! Patti

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post jinwon Main CFD Forum 3 June 14, 2007 14:50 Newbie FLUENT 0 March 1, 2004 11:24 CFD MAN Main CFD Forum 2 August 23, 2002 20:01 PattiMichelle Phoenics 7 February 13, 2002 13:38 KIAN FLUENT 1 February 29, 2000 06:54

All times are GMT -4. The time now is 18:10.